Blizzard could make it so that the dreadlords allied with the Forsaken have re-establsihed their contact with their homeworld and had their brothers join the ranks of the Forsaken, and therefore with the Horde.
Dreadlords did not ally with the Forsaken. Varimathras is the only one and he did not join by his own free will. Even so, these are the very demons the whole of Azeroth is fighting against. Why help them gain access to their home world? It will only make them a threat. Dreadlords are powerful demons in their own right, I can't imagine they'll be playable race. When ppl start rolling Nathrezim, it'll be like a Burning Legion invasion once more - you can't have Dreadlords pouring out of a newbie area and into the world. That wouldn't make any sense.
t was the dreadlords Mal'ganis and Tichondrius who first spread the taint and corruption of the Legion across Lordaeron and Felwood respectively, and after their deaths by the hands of the enemies of the Scourge, it was Varimathras and his brothers, Detheroc and Balnazzar, who were put in charge of the undead forces in Lordaeron while Archimonde took his forces across the sea to Kalimdor to siphon energy from the World Tree Nordrassil on Mount Hyjal.
They have done too much evil to be considered an ally, even to the Horde. Varimathras can't be trusted, there's no reason for him to keep serving Sylvanas that we know of - he might have his own plans to regain power altogether. No, don't think Nathrezim will become a playable race. It just wouldn't be right - no one trusts them, and there's only 1 in service of the Horde due to Sylvanas' manipulation.
Worgen are unlikely too, but easier to create a storyline for. What Blizz did to the Draenei is easily repeated for the Worgen - especially those on Azeroth, not longer in contact their home world. They would be an ally to the Horde because they too are an enemy of the Alliance. Far fetched? Yes, but not an option to be discarded.
Pandaren an issue in China? I don't know if Blizz is going to care for that - I guess they are. It would be silly not to with the amount of income at stake. They did re-do the skin on their undead, so they do respect their culture. Would pandaran get another skin in China too? They would just be a kind of bear. Would they then not still be panda's? Blizz would need to find a loophole to be able to do that, but it seems plausible.
It could be Furbolgs too, for if you did the Draenei starting area, you see that the Draenei are very close with the Furbolgs and even speak their language. Just like the Horde only has on tribe of trolls (Darkspear) as an ally, while the others are hostile to both factions, the Alliance might get a tribe of Furbolgs at their side. Not impossible imho.
Hell, a tribe Furbolg could even side with the Horde for all we know - it's all so easy to manipulate. Who would have expected Blizz to get a playable race from outer space? Nah, I'm not calling any shots yet. They might even invent some - yet unknown - amphibic tribe (naga offshoot).
Or Dark Iron dwarves, you could form them into a Horde race when you're willing to rape the lore again - that would give the Horde another Paladin race, like the Furbolgs could give alliance another Shaman race.
Ah, speculation! Gotta love it. edit: spelling Edited, Aug 25th 2008 11:26am by Hence